1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with an improved locking device for ensuring that a fastener which attaches a guide thimble to the bottom nozzle of the fuel assembly is maintained in its tightened position.
2. Description of the Prior Art
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a plurality of fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The interconnected top and bottom nozzles and elongated guide thimbles which extend therebetween form the structural skeleton of the fuel assembly. For satisfactory reactor operation, the skeleton of each fuel assembly must exhibit a high degree of structural integrity for maintaining the grids and fuel rods supported thereon in precise alignment relative to the grids and fuel rods of other fuel assemblies within the reactor core. On the other hand, in view of the current demand for reconstitution of fuel assemblies, the skeleton must be capable of easy disassembly to repair and replace any failed components of the assembly. One recent practice is to make reconstitution possible by providing the fuel assembly with removable top and bottom nozzles, for example, as illustrated and described in U.S. Pat. No. 4,323,428 to Schallenberger et al which is assigned to the assignee of the present invention. Other arrangements of fuel assemblies which can be disassembled are illustrated and described in U.S. Pat. Nos. 3,828,868 and 4,208,248 to Jabsen.
In one conventional embodiment of an arrangement for releasably attaching the guide thimbles of the fuel assembly to its bottom nozzle, screws are used to interconnect and precisely position lower ends of the guide thimbles with and upon the upper central adapter plate of the bottom nozzle. Specifically, each screw is inserted from below the bottom nozzle upwardly through one of a number of passageways through its adapter plate. The outer externally threaded end of the screw is threaded into an internally threaded plug rigidly connected on the lower end of the guide thimble. The screw has an inner enlarged head which bottoms against a ledge defined in the adapter plate by an enlarged diameter portion of the passageway. The end of the screw head has an upstanding annular wall which extends contiguous with the enlarged portion of the passageway. Then, once the screw is torqued to its tightened position, opposite diametric portions of the annular wall on its head are bulged into cut outs or recesses in the enlarged passageway portion to prevent the screw from counterrotating and unloosening the rigid attachment of the guide thimble with the bottom nozzle.
Although this particular attachment arrangement provides a satisfactory interconnection between the guide thimbles and the bottom nozzle of the fuel assembly, the screw has proven difficult and expensive to fabricate and is frequently scrapped after one-time use due to cracks being formed therein by the bulging operation for anchoring the screw against counterrotation. Consequently, a need exists for improvement of the guide thimble attachment arrangement which retains the benefits provided by the use of a screw as in the previous arrangement but avoids the disadvantages accompanying the particular embodiment thereof. While a number of different arrangements for anchoring screws and bolts in general are known in the prior art, for example as illustrated and described in U.S. Pat. Nos. 2,782,827 and 2,886,090 to Rosan, 1,814,858 to Rutter and 3,160,188 to Frank, none are viewed as particularly adapted to provide the improvement being sought herein.